Thales of Miletus: Considered the first philosopher in the Greek tradition, Thales proposed that water was the origin of all matter.

Anaximander: A contemporary of Thales, he developed the concept of the apeiron, or the "unlimited," as the fundamental substance of the universe.

Anaximenes: Also from Miletus, he suggested that air was the primary substance that could be transformed into other elements.

Pythagoras: Known for his contributions to mathematics and his belief in the transmigration of souls, Pythagoras founded a school that influenced many later philosophers.

Xenophanes: Criticized the anthropomorphic views of the gods and proposed that there is only one god, who is the mind of all things.

Heraclitus: Famous for his doctrine of change, expressed in the maxim "You cannot step into the same river twice".

Parmenides: Argued that reality is unchanging and that change is an illusion, influencing later philosophers like Plato.

Empedocles: Proposed the theory of the four classical elements (earth, water, air, and fire) and the forces of love and strife.

Democritus: Known for his atomic theory, which posited that the universe is made up of indivisible particles called atoms.